The present invention relates to a friction material for use as automotive disk brake pads and brake linings, and particularly a non-asbestos friction material which produces less low-frequency noises during braking, while keeping its low attack to the mating member and its high fade resistance and friction coefficient.
Asbestos pads are now being rapidly replaced by non-asbestos ones. One problem with such non-asbestos pads is that they tend to produce low-frequency noises during braking.
One way to reduce such noises is to rub off any deposits on the surface of the disk rotor, which are the main cause of such noises, by adding an abrasive having a Moh's hardness of not less than 6 to the friction material. Another way is, as disclosed in Unexamined Japanese Patent Publication 4-60225, to add a flaky inorganic substance having self-shearing lubricating properties such as mica and talc to the friction material. By adding such a substance, the mating member can slide smoothly on the friction member even while the surface pressure is high, at which time low-frequency noises tend to be produced. Thus, it is possible to prevent stick slip, which is the major cause of low-frequency noises.
Another solution, which is proposed in Unexamined Japanese Patent Publication 5-32956, is to add a non-metallic inorganic substance having a Moh's hardness between 3 and 9 and an inorganic hydrate having a Moh's hardness not exceeding 5 in predetermined amounts, while reducing the carbon content.
In the method in which deposits on the surface of the rotor are removed by adding an abrasive to the friction member, the abrasive has to have a sufficiently large particle size. But the larger the particle size of the abrasive, the more severely the friction member tends to attack the rotor. This leads to local increase in the surface pressure during braking. Such locally high surface pressure may cause streaks on the surface of the rotor, which tends to destabilize the braking effect and increase squeaks (high-frequency noise) during braking.
In the method in which a flaky inorganic substance such as mica or talc is added, due to crystal structure of the inorganic substance used, the friction coefficient (hereinafter referred to as .mu.) tends to be low, so that the resistance to fading drops. It is also known that if an abrasive having a large particle size is added to the friction material in an attempt to increase .mu., the resistance to fading drops markedly for unknown reasons.
In the technique proposed in Unexamined Japanese Patent Publication 5-32956, by reducing the carbon content, it is possible to suppress the production of substances that form a film deposit. Also, the hydrate added serves to prevent the film forming substances from adhering to the surface of the rotor. Moreover, the inorganic substance having a Moh's hardness of 3-9 removes any deposits on the rotor surface. But since the hydrate has a low Moh's hardness, .mu. tends to be low. Also, the hydrate presumably causes a drop in the wear resistance of the friction material.
Further, if the inorganic substance having a high Moh's hardness is too high in particle size, the rotor tends to be worn unevenly. If too low, deposits cannot be removed effectively.
The applicant of this invention also proposed in Unexamined Japanese Publication 62-15281 to add 8-12% by volume of magnesium oxide powder having a maximum particle diameter not exceeding 250 .mu.m in order to increase .mu.. Since the magnesium oxide has a high Moh's hardness, it acts as an abrasive. But if its particle size is too large, the friction member tends to more severely attack the mating member. If too small, it cannot remove deposits effectively.
In short, none of these conventional methods can suppress low-frequency noise without worsening any of the other properties of the friction material.
An object of the present invention is to provide a high-performance, non-asbestos friction material which produces less low-frequency noises while keeping low its tendency to attack the rotor, its high fade resistance and high .mu..